Polymer electrolyte membrane (PEM) water electrolysis has demonstrated its potentialities interms of cell efficiency (energy consumption 4.0-4.2 kW/Nm3 H2) and gas purity (> 99.99%2H2). Current research activities are aimed at increasing operating pressure up to several hundredbars for direct storage of hydrogen in pressurized vessels. Compared to atmospheric pressureelectrolysis, high-pressure operation yields additional problems, especially with regard to safetyconsiderations. In particular the rate of gases (H2 and O2) cross-permeation across the membraneand their water solubility both increase with pressure. As a result, gas purity is affected in bothanodic and cathodic circuits, and this can lead to the formation of explosive gas mixtures. Toprevent such risks, two different solutions, reported in this communication, have beeninvestigated. First, the chemical modification of the solid polymer electrolyte, in order to reducecross-permeation phenomena. Second, the use of catalytic H2/O2 recombiners to maintain H2levels in O2 and O2 levels in H2 at values compatible with safety requirements.

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